Summary/Abstract The proposed experiments are designed to examine how sensory selectivity is adjusted to initiate appropriate behavior. In mice, two fundamental visually-evoked behaviors, freezing and escape, are initiated respectively by the movement of a small disc that moves slowly across the visual field (sweeping), or a disc that rapidly expands to fill the visual field (looming). However, the synaptic interactions that mediate the selection of these visually-evoked responses are largely unknown. In the superior colliculus (SC), wide field vertical (WFV) cells respond to sweeping or looming stimuli and project to the thalamic pulvinar nucleus (PUL), a brain region involved in the initiation of visually-guided behavior. WFV cells receive input from the retina and V1, intrinsic SC neurons, and the parabigeminal nucleus (PBG). Based on previous studies of the avian nucleus isthmi (considered the PBG homologue), we hypothesize that SC-PBG connections gate the responses of WFV cells to their retinal input, and this stimulus selection network determines the subsequent initiation of appropriate behavioral responses. We propose to examine SC-PBG circuits in detail, and test whether manipulating their activity can shift the responsiveness of WFV cells, alter PUL responses to sweeping and looming stimuli, or impact the initiation of visually-evoked freezing and escape behavior. We will study this system at multiple levels, from the ultrastructure of synapses to behavioral assessments, using an array of innovative strategies to examine circuit function with cell type specificity. The specific goals of the proposed experiments are: to characterize the ultrastructure of synaptic input to WFV cells and synaptic connections that link the SC and PBG (Aim 1a), test how optogenetic activation of the synaptic connections between the SC and PBG affect the responsiveness of WFV and PBG cells (Aim 1b), determine whether PUL receptive field properties differ based on their unique responses to WFV cell input (Aim 2a), test whether PUL responses are altered by manipulating SC-PBG circuits (Aim 2b), and determine if chemogenetic manipulation of SC-PBG pathways affects visually-evoked freezing or escape responses (Aim 3). We believe that this integrated approach can address how sensory selectivity is adjusted to initiate appropriate behavior, while providing the precision of single cell circuit analysis.